The National Security Agency has technology capable of recording all the phone calls of an entire country and replaying them later, a report based on leaked documents says.
The Washington Post, citing documents leaked by former NSA contractor Edward Snowden, said the technology functions like a time machine by being able to reach into the past.
The report said the NSA can collect 100 per cent of the calls of a country and reach as far back as one month with the tools called MYSTIC and RETRO.
The leaked documents say the tools can "retrieve audio of interest that was not tasked at the time of the original call".
The Post said that at the request of US officials, it withheld details that could be used to identify the country where the system is being used or other countries where it may be used in the future.
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If accurate, the program would be more powerful than any other NSA program by allowing the spy agency to tap into the entire network from a country.
Dozens of documents leaked by Snowden have sparked outrage in the US and abroad about the vast capabilities of the intelligence programs.
US officials have defended the programs as needed to thwart terrorist attacks, but President Barack Obama has ordered reforms for the surveillance programs.
